A biosensor (or biological sensor) is an analytical device including a biological element and a transducer that converts a biological response into an electrical signal. Certain biosensors involve a selective biochemical reaction between a specific binding material (e.g., an antibody, a receptor, a ligand, etc.) and a target species (e.g., molecule, protein, DNA, virus, bacteria, etc.), and the product of this highly specific reaction is converted into a measurable quantity by a transducer. Other sensors may utilize a non-specific binding material capable of binding multiple types or classes of molecules or other moieties that may be present in a sample. The term “functionalization material” may be used herein to generally relate to both specific and non-specific binding materials. Transduction methods used with biosensors may be based on various principles, such as electrochemical, optical, electrical, acoustic, and so on. Among these, acoustic transduction offers a number of potential advantages, such as being real time, label free, and low cost, as well as exhibiting high sensitivity.
An acoustic wave device employs an acoustic wave that propagates through or on the surface of the specific binding material, whereby any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave. The presence of functionalization material on or over an active region of an acoustic wave device permits an analyte to be bound to the functionalization material, thereby altering the mass being vibrated by the acoustic wave and altering the wave propagation characteristics (e.g., velocity, thereby altering resonance frequency). Changes in velocity can be monitored by measuring the frequency or phase characteristics of the sensor and can be correlated to a physical quantity being measured.
There has been a growing interest in electroacoustic devices for high-frequency sensing applications due to the potential for high sensitivity, resolution, and reliability. However, it is not trivial to apply electroacoustic technology in certain sensor applications—particularly sensors operating in liquid/viscous media (e.g., chemical and biochemical sensors)—since longitudinal and surface waves exhibit considerable acoustic leakage into such media, thereby inhibiting sensing capability.
In the case of a piezoelectric crystal resonator, an acoustic wave may embody a bulk acoustic wave (BAW) propagating through the interior (or “bulk”) of a piezoelectric material. BAW devices typically involve transduction of an acoustic wave using electrodes arranged on opposing top and bottom surfaces of a piezoelectric material. In a BAW device, three wave modes can propagate, namely, one longitudinal mode (embodying longitudinal waves, also called compressional/extensional waves, and two shear modes (embodying shear waves, also called transverse waves), with longitudinal and shear modes respectively identifying vibrations where particle motion is parallel to or perpendicular to the direction of wave propagation. The longitudinal mode is characterized by compression and elongation in the direction of the propagation, whereas the shear modes consist of motion perpendicular to the direction of propagation with no local change of volume. Longitudinal and shear modes propagate at different velocities. In practice, these modes are not necessarily pure modes as the particle vibration, or polarization, is neither purely parallel nor purely perpendicular to the propagation direction. The propagation characteristics of the respective modes depend on the material properties and propagation direction respective to the crystal axis orientations. The ability to create shear displacements is beneficial for operation of acoustic wave devices with fluids because shear waves do not impart significant energy into fluids.
Certain piezoelectric thin films are capable of exciting both longitudinal and shear mode resonance, such as hexagonal crystal system piezoelectric materials including aluminum nitride (AlN) and zinc oxide (ZnO). To excite a wave including a shear mode using a standard sandwiched electrode configuration device, a polarization axis in a piezoelectric thin film must generally be non-perpendicular to (e.g., tilted relative to) the film plane. In biological sensing applications involving a liquid media, the shear component of the resonator is used because it is not damped completely by liquid loading. In this case, the piezoelectric material is grown with a c-axis orientation distribution that is non-perpendicular relative to a face of an underlying substrate to enable the shear component to be increased relative to the longitudinal component.
An electromechanical coupling coefficient is a numerical value that represents the efficiency of piezoelectric materials in converting electrical energy into acoustic energy for a given acoustic mode. Changing the c-axis angle of inclination for hexagonal crystal system piezoelectric materials causes variation in shear and longitudinal coupling coefficients. FIG. 1 embodies plots of shear coupling coefficient (Ks) and longitudinal coupling coefficient (Kl) each as a function of c-axis angle of inclination for AlN, although other piezoelectric materials show similar behavior. At certain angles (e.g., 46° and 90°) the longitudinal component is minimized and K1 has a zero value, and at certain angles (e.g., 0° and 67°) the shear component is minimized and Ks has a zero value. At all other angles of C-axis inclination, there exist both shear and longitudinal components of wave propagation. Devices built with C-axis angles that include both longitudinal and shear modes (e.g., at angles except for about 0°, 46°, 67°, and 90°) are referred to as quasi-shear mode devices.
Solidly mounted resonator BAW technology relies on a reflective structure (e.g., reflector array, acoustic mirror, etc.) underneath the resonator to help keep the energy confined within the resonating structure. In other words, the reflective structure reflects the acoustic energy back toward the resonator and isolates the resonator from the substrate. If the reflectivity of the reflective structure is not perfect, then energy will be lost from leakage into the substrate, which reduces the quality factor (Q) of the resonator. A typical reflector for a solidly mounted resonator BAW device includes alternating high and low acoustic impedance layers arranged between a substrate and a piezoelectric layer.
Quarter-wave thin-film technology is commonly used to create the reflective stack (e.g., sometimes referred to as a Bragg reflector or grating) using multiple layers of materials of different acoustic impedances. Providing alternating layers of materials with varying acoustic impedance promotes constructive interference of waves reflecting off the layer boundaries and creates a band of frequencies where high reflectivity (low transmissivity) is achieved. A typical transmissivity plot for a quarter-wave reflector design using a combination of five alternating layers of silicon dioxide [SiO2] and tungsten [W] is shown in FIG. 2. This transmissivity plot exhibits shear wave leakage, since the plot is devoid of a region in which the shear response 2A (e.g., minimum shear transmissivity) and longitudinal response 2B (e.g., minimum longitudinal transmissivity) overlap significantly. Shear wave leakage that exists with a quarter-wave design (due to the lack of overlap of the responses) can reduce the obtainable Q of the resonator.
Conventional acoustic reflectors are not well-suited to provide high Q for the shear mode of an acoustic resonator in quasi-shear mode applications, while preventing both longitudinal and shear components from reflecting off the backside of the substrate (which would interfere with measurements obtained with a sensor incorporating the resonator). Conventional acoustic reflectors tend to exhibit excess transmissivity for at least one of the shear and longitudinal modes at the desirable operating frequencies for certain acoustic resonator-based sensing applications.
Accordingly, there is a need for improved acoustic reflectors capable of enhancing reflection of both shear and longitudinal energy for quasi-shear mode sensing applications.